Summary:
Rather than specializing standard library numeric traits for `signed __int128` and `unsigned __int128`, bring these traits into folly and choose them where needed.

Note that it is undefined behavior to specialize the standard traits types.

Reviewed By: ericniebler

Differential Revision: D26506904

fbshipit-source-id: 0f7b0fa445c2713961b345b453fef8ff58032ee5